1. Field of the Invention
This invention relates to devices which display information and, in particular, to liquid crystal based devices which display information.
2. Art Background
Liquid crystal based devices are widely used to display information. These devices, in general, have at least two optical states that depend on the molecular orientation of the liquid crystal material. One of these optical states is typically associated with a spatial ordering of the molecules. This ordering is most often changed through the application of an electrical field to yield a second molecular spatial configuration, and thus, a second optical state.
The desired spatial ordering is not necessarily inherent in the liquid crystal material. Generally, the liquid crystal material must be contacted with an order inducing substance before the relatively ordered state of the liquid crystal is assumed. This order inducing substance is typically incorporated into the device cell and continually contacts the liquid crystal material. The cell with its ordering substance and liquid crystal material is generally constructed by forming electrodes on a relatively transparent material, e.g., a glass plate. (The electrodes are employed to apply the necessary fields for optical state switching.) At least a portion of the transparent material on the electrode surface is then covered with the ordering substance. (Although for nematic liquid crystal devices the entire transparent material is covered, it has been reported that ferroelectric liquid crystal devices only operate properly when a polyester ordering material is kept from covering the region of the transparent material where information is ultimately to be displayed. See Kondo et al Japanese Journal of Applied Physics, 22(2), L85 (1983).) The two treated transparent media with the ordering material on each facing the other are spaced a desired distance, e.g., a distance typically in the range 0.5 to 100 .mu.m, and the region between the plates filled through conventional techniques such as capillary action and vacuum filling.
Typically for cells relying on liquid crystal materials in a nematic phase, highly cross-linked polyimides have been utilized to establish the desired ordering. For example, PI 2555, a cross-linking polyimide sold by E. I. DuPont De Nemours and Company, Incorporated, when fully cured has been utilized to align liquid crystal material such as 4-n-pentyl-4-cyano-biphenyl. The resulting liquid crystal cells have been utilized extensively in a variety of applications such as for watch and calculator displays. However, the success of ordering nematic phase liquid crystal materials has not been extended to the ordering of liquid crystal materials in a smectic phase. Thus, devices relying on smectic liquid crystals such as those reported by Clark et al in Applied Physics Letters, 36, 899 (1980) have had contrasts over display size areas significantly less than those typically achieved with nematic based devices.